One of the most intensively studied anode materials for lithium batteries is Si due to its relatively high theoretical capacity (4200 mA/g, ca. Li4.4Si). In spite of the high capacity, silicon exhibits a large volume change (>300%) upon lithium insertion and extraction, which causes pulverization and breakdown of the electrical conductive network, resulting in rapid capacity decay and a rapid decrease in cycling stability.
Recently, many studies have focused on reducing such volume change via coating silicon with lithium ion conducting active carbon phase to prevent the particle aggregation when the particle is pulverized. Various methods, such as pyrolysis or chemical vapor deposition (CVD), ball milling or mechanical milling, chemical reaction of gels, and dehydration of a carbon precursor, have been employed for preparing carbon coated silicon composite. From the viewpoint of uniform structure of carbon layer, CVD is a potential method for lithium ion batteries.
On the other hand, porous structure is an effective way to accommodate the volume change. Some approaches to incorporate porous structures as a buffer zone for volume change, demonstrate another means of accommodating the volume expansions/contractions.
Rongguan Lv et al, in “Electrochemical behavior of nanoporous/nanofibrous Si anode materials prepared by mechanochemical reduction”, Journal of Alloys and Compounds, 490 (2010), pp. 84-87, prepared a mixture of nanoporous and nanofibrous silicon (NPNF-Si) by a mechanochemical reaction between SiCl4 and Li13Si4 under ball-milling. A nanofibrous and nanoporous structure can be obtained. However, the reversible capacity is relatively low (746.6 mAh·g−1) and the capacity was decreased rapidly after 30 cycles.
Hyunjung Kim et al., in “Three-Dimensional Porous Silicon Particles for Use in High-Performance Lithium Secondary Batteries”, Angewandte Chemie-International Edition, 2008, 120, 10305-10308, reported a template method for the formation of 3D porous bulk Si particles which delivered a reversible capacity of 2800 mAh·g−1 at a rate of IC after 100 cycles. The cycling improvement benefits from its highly porous and interconnected structure.
However, the synthetic process is too complicated and expensive. In addition, the method uses a large amount of strong corrosive acid, e.g. hydrofluoric acid, which is toxic and expensive.
There remains a need for a silicon/carbon composite for use as lithium ion battery anode material having excellent capacity and cycling stability, and a soft and simple process of preparing such a silicon/carbon composite using no corrosive acid.